1. Field of the Invention
The present invention relates to an image sensing device for converting optical signals to electrical signals, and specifically relates to an image sensing device which logarithmically converts a photoelectric current generated in accordance with the intensity of incident light for output.
2. Description of the Related Art
Color image sensing devices are provided with red, green, and blue filters, through which passes light which impinges photoreceptor sensors of red (R), green (G), and blue (B). However, these filters have different transmittance, e.g., the amount of light transmitted by each filter is different in the case of achromatic color. Specifically, the transmittance of red color is greatest, followed by green, then blue in that order. Accordingly, white balance adjustment is performed to correct differences in the amount of said transmitted light.
When sensors having linear characteristics output linear photoelectric current in accordance with the intensity of incident light, differences in the amount of light impinging the R, G, and B photoreceptors produce input/output (I/O) characteristics as shown in FIG. 1(a) (wherein the horizontal axis represents the linear scale). In the case of sensors having linear characteristics, adjustments are made to the gain of the amplification circuits connected to the output side of the sensors, so as to match R, G, B characteristics and correct white balance by changing the slope of the I/O characteristics (refer to U.S. Pat. No. 4,734,762).
On the other hand, U.S. Pat. No. 5,241,575 discloses an image sensing device provided with sensors which logarithmically convert a photoelectric current generated in accordance with the intensity of incident light. When the intensities of the incident light impinging the R, G, and B photoreceptors of this image sensing device are different, the I/O characteristics of these sensors are as shown in FIG. 1(b) (wherein the horizontal axis represents the logarithm scale). In the case of an image sensing device of this logarithm conversion type, adjustments of the gain of the amplification circuits cannot accomplish matching of the R, G, B characteristics. Accordingly, white balance adjustment is accomplished after analog-to-digital (A/D) conversion of the sensor output signals. Thus, in conventional image sensing devices of the logarithm conversion type, there are differences in the output levels of R, G, and B sensors, such that n bits less than the number of bits of A/D conversion are allocated as gradient numbers relative to amplitude A of an image signal.